In the processing of semiconductor devices, photoresist materials are used for a variety of applications, including use as etch masks for the fabrication of surface features and implant masks for patterning implants in a semiconductor material. Understandably, significant problems arise if the photoresist material is missing or improperly exposed and/or developed. Without early detection that would permit the use of remedial procedures, semiconductors with improper photoresist coverage must be scrapped at considerable cost.
In response to this problem, the prior art has sought to visually inspect semiconductor wafers to ensure the presence and suitability of their photoresist coverage. For this purpose, the prior art has proposed the use of fluorescence microscopes operating at a wavelength which will cause the photoresist material present on a wafer to fluoresce, though only sufficiently to be detectable with considerable magnification, e.g., about 100.times. or more. With this method, each wafer is individually placed on a microscope stage, an operator focuses the microscope to obtain a clear image of the wafer, and then illumination radiation is projected along the optical axis of the microscope and focused on the surface of the wafer. The operator must then scan the surface of the wafer to visually determine the presence of the photoresist and evaluate its coverage and patterning.
A single silicon wafer is likely to go through numerous masking levels, an error at any one of which can result in wafer scrap. Because deficient photoresist coverage, exposure and development occur generally randomly at low levels, a 100% inspection rate is often necessary to achieve any significant reduction in wafer scrap. Unfortunately, doing so using the above-noted prior art methods entails significant additional labor and slows processing considerably. Therefore, it would be desirable if an inspection method were available that was capable of accurately and reliably detecting the presence and patterning of a photoresist material on the surface of a substrate, though at a rate vastly higher than that possible with prior art techniques.